1. Field of the Invention
This invention generally relates to an array antenna system and, more particularly, to a method and system for providing a desired radiation pattern in a desired direction and for providing a desired sidelobe pattern by using a plurality of antenna elements such as arranged in a phased array antenna.
2. Prior Art
In conventional antenna systems having a plurality of antenna elements aligned in line or uniformly arranged on a plane, each of the antenna elements is connected to a phase shifter for changing the phase of a signal and an amplitude controller for adjusting the amplitude level by amplifying or attenuating the signal amplitude. The phase shifter controls the phase of the signal transmitted from or received by each of the antenna elements so that the direction of a beam emitted by the antenna system is steered and the beam of the antenna system is scanned. The amplitude levels of the antenna elements are adjusted by the amplitude controllers in such a manner that the amplitude distribution given by the antenna elements corresponds to a predetermined distribution, for example a Taylor pattern, whereby a resultant radiation pattern obtained by the antenna elements is provided with a desired sidelobe level.
Recently, in addition to the antenna system in which the antenna elements are arranged in line or on a plane, there has been an increasing desire for antenna elements to be disposed on a nonplanar surface with a radiation pattern having a desired sidelobe level. FIG. 1 shows an example of such a nonplanar arrangement which was shown in TOKUNAGA et al. "A Cylindrical Array Antenna for SSR Mode-S" IEICE National Convention. Optics & Radio Wave Division, 1986, p. 82. In FIG. 1, antenna elements 1 are arranged on a cylinder surface and connected to phase shifters 2 which are controlled by a phase shifter controller 3. The phase shifters 2 are connected to amplitude controllers 4 which adjust the amplitude of signals received by the antenna elements 1. The signals from the amplitude controllers 4 are combined by a power combiner 5 and supplied to a receiver 6.
In such an antenna system as mentioned above, the phase of each of the phase shifters 2 is changed by the phase shifter controller 3 so that a beam is scanned for carrying out receiver operations. At the same time, it is necessary for the amplitude level given through each of the amplitude controllers 4 to be appropriately adjusted in order to obtain a desired sidelobe level. An approach taken to the adjustment of the amplitude level in the prior art wast to, first, compute a number of resultant radiation patterns produced from the combination of the antenna elements 1 by using a plurality of parameters such as the arrangement shape of the antenna elements 1, the active sector angle, and value of edge-taper of the amplitude distribution supplied to the antenna elements 1. Next, selected is a readiation pattern having a desired sidelobe level, and then the amplitude level of each of the amplitude controllers 4 is set in accordance with the parameters for the selected pattern.
Since the prior art antenna systems are constructed as mentioned above, a number of arithmetic operations using a plurality of paramenters need to be carried out to obtain a radiation pattern having a desired sidelobe level in antenna systems in which antenna elements are arranged on a curved surface.
FIG. 2 shows another prior art which are disclosed in HARIU et al. "Formation of Low Sidelobe Pattern in Conformal Array Antenna" IEICE Spring National Convention, 1988, B-118. The array antenna shown in FIG. 2 includes antenna elements A.sub.1, A.sub.2, . . . A.sub.M arranged on a curved surface, phase shifters Pa.sub.1, Pa.sub.2, . . . Pa.sub.M, amplitude controllers At.sub.1, At.sub.2, . . . At.sub.M. In order for the array antenna to provide a desired sidelobe level, each of the antenna elements must be provided with an appropriate excitation amplitude. The excitation amplitude A.sub.i (i=1, 2, . . . M) is given by the following equation: EQU A.sub.i =T.sub.i .multidot.W.sub.i /E.sub.i (.theta..sub.i)(1)
Where, T.sub.i represents the amplitude given to the antenna element of element number i when the antenna elements of the conformal array antenna are projected on a plane perpendicular to the main beam axis, W.sub.i represents a weighting factor for compensating the density of antenna elements on the projected plane, and E.sub.i (.theta..sub.i) represents the amplitude of antenna element No. i's pattern in the main beam direction.
Since the excitation amplitude A.sub.i is determined as mentioned above in the prior art array antenna excitation, when a Taylor pattern is used for T.sub.i, the positions of nulls of the actual pattern are different from those of the ideal Taylor pattern. As a result, the desired side-lobe level may not be obtained.